Vernier rheostat



Oct 1939- w.-G. MOALLISTER ET AL 2,176,090

VERNIER RHEOSTAT Filed June 8, 1936 INVENTORS VVaZier M AZZL'S fer 0.Sfewar Michael BY QM m 1 EM ATTORN EYS Patented Get. 17, 1939 UNITEDSTATES VERNIER RHEOSTAT Walter G. McAllister and Oscar Stewart Michael,

Foxboro, Mass, assignors to The Foxboro Company, Foxboro, Mass,

chusetts a corporation of Massa- Application June s, 1936, Serial No.84,032

6 Claims.

This invention relates to rheostats, and more inparticular to those foruse in controlling and recording instruments.

An object of this invention is to provide a rheostat having alow-resistance element and a highresistance element, both of which maybe manipulated for current-controlling purposes.

Another object is to provide a rheostat in which the electrical circuitincludes only two movable contacts, the other connections in the circuitbeing fixed.

The above and other objects will be apparent from the detaileddescription and drawing wherein the prefeired embodiment of thisinvention is illustrated.

In the drawing:

Figure 1 is a top view with the operating knob removed so as to show thebase member and certain elements mounted thereon;

Figure 2 is a sectional view on the line 22 of Figure 1;

Figure 3 is a sectional view on the line 3-3 of Figure 2;

Figure 4 is a diagrammatic view of the device for the purpose ofexplaining its operation.

This embodiment comprises, broadly, a base member of insulating materialwhereupon a cupshaped shell made of any suitable material is mounted asan enclosing and dust-tight cover.

The base member acts as a mounting for the resistance elements, bindingposts and operating 'shaft. One end of the operating shaft may, for

convenience, extend through the bottom of the shell. Secured to the endof the shaft where it extends above the base member is a usual operatingknob 8.

Referring to Figure 2, base I provides mounting means for the rotatableelements, which comprise the shaft 5, surrounded by and fixed torotatable sleeve 4 which is centrally journaled in the base I. The upperend of sleeve 4 has a collar 6 wherein is mounted a screw 1 to preventthe shaft from rotating relatively to the sleeve. Sleeve 4 is reduced onits lower end to provide a shoulder 9, upon which is fixed a disc I0.Thread ed upon the end of sleeve 4, the nut II retains disc I!) inplace. Surrounding sleeve 4, below collar 6, is a spring washer I2 whichis arranged to exert pressure downwardly upon the upper surface of baseI and upwardly upon the lower surface of collar 6.

The base I, extending into shell 2, is shapedbearing I4 and extendedcollar I5. Due to the tendency of spring washer I2 to bias rotatablesleeve 4 upwardly, disc II] is held tightly against the lower fiat endof the extended collar I5. Mounted between annular bearing l4 and discIll, and journaled upon extended collar I5, is upper disc I6. U per discI6 is provided on its under side with hub I! which retains the two discsin spaced relationship.

As best shown in Figures 2 and 3, mounted upon upper disc I6 andextending to points near lower disc I!) are two pins I8 and I8. In alike manner, mounted upon the lower disc Ill and extending to a pointbetween pins I8 and I8 is similar pin I9. These three pins are mountedin line upon an arc of a circle which has its center at the center ofrotation so that pin i9 may move freely to any point between pins I8 andi8, but may not be moved beyond these pins.

As best shown in Figure 1, spaced from the edge of base I and extendingtherethrough are binding posts 20 and 2| to which connecting wires (notshown) may be attached. On the under side of base I (see Figures 2 and3), and attached to binding posts 20 and 2!, are segment-shaped brackets22 and 23 which provide mounting means for fixed spring brushes 24 and25, respectively.

Brushes 24 and 25 are made of thin, arc-shaped strips of resilientmetal; and extend from brackets 22 and 23 in opposite directions topoints which, in the present embodiment, are spaced radiallyapproximately apart. Upon the free ends of brushes 24 and 25 are contactportions 24 and 25', respectively.

Mounted in a groove about the periphery of the lower disc I0 islow-resistance element 26. This resistance element is formed ofresistance Wire coiled upon and insulated from a Wire of largerdiameter. Mounted in a groove about the periphery of the upper disc ishigh-resistance element 21. The structure of element 27 is similar tothe structure of element 26, but diifers in that the resistance wire isof higher resistance. As best shown in. Figures 3 and 4, theseresistance elements are each attached at one end to a flexibleconnecting wire 28 which is coiled about hub I1 and is of a lengthsufiicient to allow limitedrelative movement between the two resistanceelements.

Contact portions 24 and 25 are held under tension against theperipheries of resistance e1ements 26 and 21, respectively, due todistortion of the resilient spring portions of the brush elements. Asbest shown in Figure 1, integral with 55 the top of base I, and adjacentcollar 6, are stop members 29 which provide two limits of adjustment.Threaded into a hole in the side of collar 6, in a position so as to beengageable with stops 29, is screw 30. During manipulation of therheostat, the engagement of screw 30 with stops 29 prevents rotation ofthe elements beyond either of these two limits of adjustment. Shell 2 isretained in place by screws 3|, and is provided at the bottom (seeFigure 2) with holes 32 which may be used to secure the rheostat inplace.

The connection between the two resistance elements is such that rotationof the two elements in a counter-clockwise direction (Figure 4) willincrease the portions of both of them in the circuit.-

The operation of the device is better understood by reference to thediagrammatic showing of Figure 4, wherein the various elements have beengiven reference characters corresponding to the reference characters inthe device described above. In Figure 4, pin l9 corresponds to pin l9and is attached to a segment of disc IIJ. Assuming the rheostat to be inthe position shown in Figure 4 and it to be desirable to increase theresistance in the circuit, the shaft 5 will be turned in thecounter-clockwise direction. The first movement will merely turn thelow-resistance element 26, and the change in resistance will be verysmall. Upon further movement in the same direction, pin l9 engages pinI8; and high-re sistance element 21 is thus picked up and carried alongwith the low-resistance element. This turning increases the resistancerapidly. When slightly more than the desired resistance is in thecircuit, rotation is stopped. Backward or clockwise turning of the shaftwill now move the low-resistance element, and the high-resistanceelement will remain stationary. In this way the resistance can be slowlydecreased to the desired value. If turning the shaft in the clockwisedirection is continued, pin I9 will engage pin l8, picking up andcarrying with it the high resistance element, with the result that theresistance will be decreased rapidly. Due to this construction, rapidchange in resistance may be obtained and, at the same time, slow andaccurate adjustment can be made with the use of a single knob and shaft.

In one embodiment of this invention, a highresistance element 21 waschosen with a resistance of five times the resistance of thelow-resistance element 26. Thus, the amount of change of resistance fora given rotation is one-fifth as great in the low-resistance element asin the high-resistance element. Pins l8, l8 and I9 are so spaced thatlow-resistance'element 26 may move approximately 45 free fromhigh-resistance element 21. This movement of lowresistance element 26 of45 causes the same change in total resistance in the circuit as wouldoccur during a movement of high-resistance element 21 approximately 9.This has been found to give efficient, accurate adjustment of theresistance.

It is to be understood that the positions shown for pins I8 and I8, andthe ratio assigned to the resistance values of the two resistanceelements, are illustrative only; and that changes therein may be madefor obvious reasons.

With the construction described, it is possible, if desired, to placeresistance element 26 on sleeve 4 in such a position with respect tostop screw 30 that when shaft 5 is turned to the position of leastresistance there still remains a predetermined desired amount of theresistance 21 in the circuit in which the rheostat is connected.

This rheostat is useful where contact resistance and insulationdifficulties must be avoided, as there are only two open connections inthe circuit, and the entire device, except the shaft and the electricalcircuit, would normally bemade from an insulating material.

As various embodiments might be made of this invention, and as variouschanges might be made in the construction herein described, all withoutdeparting from the scope of the invention, it is to be understood thatall matter herein set forth or shown in the accompanying drawing is tobe interpreted as illustrative and not in a limiting sense.

We claim:

1. In a rheostat, a pair of resistance elements rotatably mountedtogether and electrically con nected by a direct flexible connection, afirst resistance element of said pair being mounted upon and fixed to ashaft and a second resistance element of said pair being free to rotateindependently of said shaft, fixed contacts to bear against saidresistance elements at points radially spaced approximately from eachother, and interengaging means upon said first and second elements tomove said second element so that said elements may be moved together orso that said first element may be given a predetermined .movement whilesaid second element remains stationary.

2. In rheostat construction, the combination of, a rotatable shaftstructure, a first disk member carrying a resistance unit fixedlymounted upon said shaft structure, a second disk member carrying aresistance unit mounted to rotate about said shaft structure, meanselectrically connecting said resistance units together, means holdingsaid disks in definite parallel rotatable relationship, a pair ofabutment portions upon one of said disk members, and a single abutmentportion upon the other of said disk members, said abutment portionsbeing in circumferential align- -ment with respect to said shaftstructure, said single abutment portion being between said pair ofabutment portions and positioned so that when one of said disks isrotated with respect to the other disk said single abutment Willcontactone of said pair of abutment portions.

3. In rheostat construction, the combination of, an adjusting means, afirst res stance unit fixedly mounted upon said adjusting means, asecond resistance unit mounted adjacent said first resistance unit,means electrically connecting said resistance units together, fixedcontacts to bear against said resistance units, means holdng saidresistance units in definite relationship so that they may be movedalong adjacent paths, a pair of projecting portions upon one of saidresistance units, a single projecting portion upon the other of saidresistance units, said single projecting portion extending between saidpair of projecting portions so that when one of said disks is rotatedwith respect to the other disk said single abutment will contact one ofsaid pair of projecting portions.

4. In apparatus of the class described, the com bination of, a rotatableshaft structure, a first frame structure carrying a first resistanceunit fixedly mounted upon said shaft structure, a second frame structurecarrying a second resistance unit mounted to rotate about said shaftstructure, flexible means electrically connecting said resistance unitstogether, means holding said frame constructions in definite parallelrotatable relationship, fixed contacts to bear against said resistanceunits at points radially spaced approximately degrees from each otherand so related to said resistance units as to have constantly changingelectrical connections with said resistance units as said frameconstructions are rotated, interengaging means upon said first andsecond frame constructions -to move said second frame constructionwhereby limited free movement may be imparted to said first frameconstruction without movement of said second frame construction, andsaid second frame construction is moved with said first frameconstruction when said first frame construction is moved in onedirection beyond that limit of said free movement, the electricalcircuit being from one of said fixed contacts, through a variableportion of one of said resistance units, said flexible means, and avariable portion ofthe other of said resistance units to the other ofsaid fixed contacts, in the order named.

5. In apparatus of the class described, the combination of, a shaftstructure, a first disc member carrying a resistance unit and mountedupon said shaft structure, a second disc member carrying a secondresistance unit mounted to rotate about said shaft structure, a frameconstruction rigidly'carrying a pair of contact members, one of saidpair of contact members contacting said first resistance unit and theother of said'pair of contact members contacting said second resistanceunit, means-electrically connecting said resistance units together,means holding said discs in definite parallel rotatable relationship, apair of abutment portions upon one of said disc members, and abutmentmeans upon the other of said disc members, said abutment means and saidabutment portions being in circumferential alignment with respect tosaid shaft structure, said abutment means being between said pair ofabutment portions and positioned so that when one of said discs isrotated with respect 3 to the other of said discs said abutment meanswill contact one of said pair of abutment portions and the adjustment ofsaid other of said discs will not be disturbed except when said abutmentmeans and one of said abutment portions .are in engaging position.

6. In apparatus of the class described,the combination of, a firstvariable resistance unit, a second variable resistance unit, each ofsaid units comprising a disc carrying a resistance element and acooperating contact member electrically engaging said resistanceelement, and mounting means holding'said resistance units in definiteparallel relationship and including an adjusting means to impartrelative movement between each of said resistance elements and itscooperating contact member to adjust the value of the effectiveresistance of each of said units, said adjusting means comprising adirect mechanical connection to said first unit and a mechanicalconnection between said first unit and said second unit to permit thedirect adjustment of said first unit and the adjustment of said secondunit through said first unit, said mechanical connection between saidfirst unit and said second unit comprising a first abutment means uponone of said units having two abutment surfaces and a second abutmentmeans upon the other of said units, said second abutment means and saidabutment surfaces of said first abutment means being in circumferentialalignment with respect to the axis of said units, and said secondabutment means being positioned between said abutment surfaces, in thesense that when adjusting movement of an amount dependent upon therelative adjustment of the two units is imparted to said first unit saidsecond abutment means will be moved into contact with one of saidabutment surfaces to thereby impart concurrent movement to said secondunit.

WALTER G. McALLISTER. 0. STEWART MICHAEL.

